Electrosurgery typically utilizes the application of high frequency currents to cut or ablate tissue structures, either utilizing a monopolar or bipolar configuration. Monopolar configurations utilize an instrument having a single electrode and rely on external grounding of the patient whereas bipolar configurations utilize both an active and return electrode on the instrument itself for application of a current between the electrodes.
Electrosurgical procedures and techniques are particularly useful in reducing patient bleeding and trauma typically associated with surgical procedures. However, the radio frequency (RF) currents applied by electrosurgical instruments are typically controlled by utilizing control signals indicative of calculated root-mean-square (RMS) voltage and RMS current values. Generally, a current sensing transformer is used to measure the amount of RF current passing through the ablation electrode such that this measured current may be used to derive the RMS current via a signal converter which first squares the RF current input signal and then averages the squared signal over a prescribed period of time. The signal converter then calculates the square root of the average squared value to result in the RMS current. Accordingly, the RMS current signal may take the form of a relatively slowly varying signal compared to a rapidly varying RF current input signal.
Likewise, a voltage sensing transformer may be used to derive the RMS voltage via a RMS voltage converter which squares the RF voltage input signal and then averages it over the same prescribed period of time. The signal converter may then calculate the square root of the average squared voltage values to result in the RMS voltage. These RMS values may be used to control operation of the power supply to maintain the RF output voltage within a desired range or at a constant value or to control the power delivered through the ablation electrode. Such control thus allows for the physician to ablate or coagulate tissue in a controlled manner and may also serve as rudimentary inputs to control algorithms for other instruments.
However, utilization of these RMS values fails to consider the changes in wave shape of the applied voltage and current levels as the device enters different operating modes, particularly in thermal modes (non-ablative) and plasma modes (ablative) thus potentially resulting in the inaccurate application of voltage to the treated tissue.